We use the mail header “Expires” frequently. It allows an author/sender of an email to specify a date/time when an email becomes outdated. It is very helpful for automatisation tasks and to manage mails (e.g. delete them or archive them) after they became outdated / reached their expiry data and to quickly make a differnce in the GUI between outdated and current mails.

The format of this header in an email is as follows (example):
Expires: Fri, 19 Jul 2024 22:39:00 +0200

In the GUI Outlook simply displays the email in grey colour and striked through in its inbox once the expiry date/time is reached.

This header is official part of RFC 4021, but unfortunately emClient doesn't recognize it or make use of it.

Please follow the official RFC 4021 and make eMClient support this header.